The highest speed and therefore most commonly employed photographic elements are those which contain a radiation-sensitive silver bromide or bromoiodide emulsion layer coated on a support. Although other ingredients can be present, the essential components of the emulsion layer are radiation-sensitive silver bromide microcrystals, optionally containing iodide, commonly referred to as grains, which form the discreet phase of the photographic emulsion, and a vehicle, which forms the continuous phase of the photographic emulsion.
Interest in silver halide photography has recently focused on tabular grain emulsions, particularly thin intermediate and high aspect ratio tabular grain emulsions. It has been shown that these emulsions can produce a variety of photographic advantages, including increased sharpness, improved speed-granularity relationships, increased blue and minus-blue speed separations, more rapid developability, higher silver covering power when fully forehardened, reduced crossover in spectrally sensitized dual coated (also referred to as two sided or Duplitized.RTM.) radiographic formats, and various imaging advantages in dye image transfer film units. Research Disclosure, Vol. 225, Jan. 1983, Item 22534, is considered representative of these teachings. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England.
In precipitating thin tabular grain silver bromide and bromoiodide emulsions, it is recognized that the bromide ion concentration in solution at the stage of grain formation must be maintained within limits to achieve the desired tabularity of grains. As grain growth continues, the bromide ion concentration in solution becomes progressively less influential on the grain shape ultimately achieved.
For example, Wilgus et al U.S. Pat. No. 4,434,226 teaches the precipitation of high aspect ratio tabular grain silver bromoiodide emulsions at bromide ion concentrations in the pBr range of from 0.6 to 1.6 during grain nucleation, with the pBr range being expanded to 0.6 to 2.2 during subsequent grain growth. Kofron et al U.S. Pat. No. 4,439,520 extends these teachings to the precipitation of high aspect ratio tabular grain silver bromide emulsions. pBr is defined as the negative log of the solution bromide ion concentration.
Daubendiek et al U.S. Pat. No. 4,414,310 describes a process for the preparation of high aspect ratio silver bromoiodide emulsions under pBr conditions not exceeding the value of 1.64 during grain nucleation.
According to Maskasky U.S. Pat. No. 4,713,320, in the preparation of high aspect ratio silver halide emulsions the useful pBr range during nucleation can be extended to a value of 2.4 when the precipitation of the tabular silver bromide or bromoiodide grains occurs in the presence of gelatino-peptizer containing less than 30 micromoles of methionine (e.g., oxidized gelatin) per gram.
Sugimoto German OLS 3,644,159 A1 (published July 2, 1987, priority Japan Dec. 12, 1985) discloses a process of preparing tabular grain silver bromoiodide emulsions in which the iodide ion concentration in the reaction vessel is maintained within a mathematically stated relationship to the bromide ion concentration. In some of the examples the pBr during nucleating is greater than 2.3.
Saitou et al German OLS 3,707,135 A1 (published Sept. 10, 1987, priority Japan Mar. 6, 1986) discloses tabular grain emulsions in which hexagon projected area tabular grains having two parallel twin planes and a neighboring edge ratio of 2:1 or less account for at least 70 percent of the total grain projected area and are monodispersed. Some of the examples of silver bromide tabular grain emulsions are prepared with a pBr during nucleation of greater than 2.3.
M. J. Harding, "The Application of Calculations of Complex Solubility to the Interpretation of Crystal Grown Phenomena in Silver Halide Emulsions", The Journal of Photographic Science, Vol. 27, 1979, pp. 1-12, plots the log of silver bromide solubility versus pBr, but was unable to correlate crystal habits observed with solution species.